1. Field of the Invention
This invention relates to a method of producing a semiconductor wafer suitable for use in solid imaging devices or the like, particularly a semiconductor wafer capable of effectively suppressing heavy metal contamination, which is a cause of white defects, even when the wafer has a large diameter of not less than 300 mm.
2. Description of the Related Art
Recently, a high-performance solid imaging device using a semiconductor is mounted onto a mobile phone or a digital video camera, and hence the performances such as number of pixels and the like are dramatically improved. As the performance to be expected in the consumer solid imaging device are a higher pixel count and an ability of taking moving images, and further miniaturization is required. In order to take moving images, it is required to combine the imaging device with a high-speed computing device and a memory device. For this end, a CMOS image sensor facilitating System on Chip (SoC) is used and the downsizing of the CMOS image sensor is developed.
A dark leakage current of a photodiode becomes a problem as a factor of deteriorating the imaging properties of the solid imaging device. The cause of the dark leakage current is heavy metal contamination during processing steps. In order to suppress the heavy metal contamination, it is attempted to form a gettering sink for heavy metal at an inside or a back face of a semiconductor wafer.
As a means for forming the gettering sink at the inside of the semiconductor wafer is mentioned, for example, a method of subjecting a semiconductor wafer to a heat treatment to form an oxygen precipitation portion inside the semiconductor wafer. In this method, however, a long-time heat treatment is required for forming the given oxygen precipitation portion, so that there are feared the increase in production cost and heavy metal contamination at the step of the heat treatment.
As a means for forming the gettering sink at the back face of the semiconductor wafer is mentioned, for example, a method of forming a polycrystalline silicon layer on the back face of the semiconductor wafer and using such a back face as a gettering sink. Especially when the semiconductor wafer has a large diameter of 300 mm or the like, however, it is difficult to form the gettering sink on the back face thereof since such a large-diameter wafer is usually a wafer to be polished on both surfaces thereof.
Therefore, it is desired to develop a method wherein a gettering sink is formed inside a semiconductor wafer without requiring a long-time heat treatment even when the wafer has a large diameter of not less than 300 mm.